Application of the suggested ice strength coefficients in ISO 19906 to intermittent crushing

Abstract

For offshore wind turbines on monopile or jacket foundations without ice cones, one of the relevant design load cases is that of ice floes or level ice crushing against the structure resulting in ice-induced vibrations. In relation to that design load case, a relevant question is which ice strength coefficient to use in the crushing formula in ISO 19906 for determining design peak loads during intermittent crushing. Despite the guidelines in the standard being relatively clear on this matter, there often exists uncertainty regarding if and how to account for velocity effects and compliance effects when defining the ice strength coefficient CR. Ice tank tests were recently conducted to investigate the dependence of global peak loads on far-field ice speed for both rigid and compliant structures. Those tests revealed that the compliance effect and velocity effect on the global loads originate from the same strengthening effect in the ice. As a consequence, the absolute global loads on the rigid structure and compliant structure did not differ significantly. Applying these results to the challenge of defining the ice strength coefficient for intermittent crushing, it can be stated that if the velocity effect is accounted for in the ice strength coefficient, then there is no need for further increase due to compliance of the structure. ISO 19906 provides some suggested values for the ice strength coefficient which include provisions for the velocity effect and can therefore be directly applied to determine the peak loads during intermittent crushing, as the standard also suggests.